Wide-angle transmission analyzer for polarized neutrons using equiangular spirals

Abstract

Much progress in neutron spectroscopy has been achieved with the invention of direct geometry time-of-flight (TOF) spectrometers, which allow an efficient determination of the scattering function S(Q,E) over large ranges in momentum Q and energy E. For separating (i) magnetic from nuclear and (ii) incoherent from coherent scattering, polarization analysis of the scattered neutrons is often required. Typically, the polarization is analyzed by means of 3He spin filters or by curved magnetized mirrors, which are arranged radially around the sample reflecting the spin-up neutrons while the spin-down neutrons are absorbed. In the following we describe the design of a wide-angle polarization analyzer that uses polarizing mirrors with an equiangular profile in transmission, i.e. the spin-down neutrons are transmitted to the detector while the spin-up neutrons are reflected and finally absorbed. Because of the almost constant angle of reflection the bandwidth is large, i.e. it is given by the ratio QP=mmax∕mmin, where mmax and mmin≃0.68 are the maximum and minimum useful m-values of the polarizing supermirror. For small samples and mmax=5, QP=7.4 is achieved. The proposed design eliminates the significant losses of neutrons due to absorption by the front faces of the polarizing blades of wide-angle analyzers in reflection. Moreover, in transmission, the phase space of the neutrons is barely affected by the mirrors and the polarization is rather constant.